Digital goods are often distributed to consumers over private and public networks—such as Intranets and the Internet. In addition, these goods are distributed to consumers via fixed computer readable media, such as a compact disc (CD-ROM), digital versatile disc (DVD), soft magnetic diskette, or hard magnetic disk (e.g., a preloaded hard drive).
Unfortunately, it is relatively easy for a person to pirate the pristine digital content of a digital good at the expense and harm of the content owners—which includes the content author, publisher, developer, distributor, etc. The content-based industries (e.g., entertainment, music, film, etc.) that produce and distribute content are plagued by lost revenues due to digital piracy.
Modern digital pirates effectively rob content owners of their lawful compensation. Unless technology provides a mechanism to protect the rights of content owners, the creative community and culture will be impoverished.
“Digital goods” is a generic label for electronically stored or transmitted content. Examples of digital goods include images, audio clips, video, multimedia, software, and data. Digital goods may also be called a “digital signal,” “content signal,” “digital bitstream,” “media signal,” “digital object,” “object,” and the like.
Watermarking
Watermarking is one of the most promising techniques for protecting the content owner's rights of a digital good. Generally, watermarking is a process of altering the digital good such that its perceptual characteristics are preserved. More specifically, a “watermark” is a pattern of bits inserted into a digital good that may be used to identify the content owners and/or the protected rights.
Generally, watermarks are designed to be invisible or, more precisely, to be imperceptible to humans and statistical analysis tools.
A watermark embedder (i.e., encoder) is used to embed a watermark into a digital good. A watermark detector is used to detect (or extract) the watermark in the watermarked digital good. Watermark detection is often performed in real-time even on small devices.
Blind Watermarking
To detect the watermark, some watermarking techniques require access to the original unmarked digital good or to a pristine specimen of the marked digital good. Of course, these techniques are not desirable when the watermark detector is available publicly. If publicly available, then a malicious attacker may get access to the original unmarked digital good or to a pristine specimen of the marked digital good. Consequently, these types of techniques are not used for public detectors.
Alternatively, watermarking techniques are “blind.” This means that they do not require access to the original unmarked digital good or to a pristine specimen of the marked digital good. Of course, these “blind” watermarking techniques are desirable when the watermark detector is publicly available.
Conventional Watermarkinig Technology
Conventional technologies for watermarking media signals rely on the imperfections of human perceptions (e.g., the human auditory system (HAS) or the human visual system (HVS)). For example, in the realm of audio signals, several conventional secret hiding techniques explore the fact that the HAS is insensitive to small amplitude changes—either in the time or frequency domains—as well as insertion of low-amplitude time-domain echoes.
The watermark can be regarded as an additive signal w, which contains the encoded and modulated watermark message b under constraints on the introduced perceptible distortions given by a mask M so that:x=s+w(M).
Commonly-used conventional watermark embedding techniques can be classified into spread-spectrum (SS) (which is often implemented using additive or multiplicative techniques) and quantization-based watermarking schemes.
Those of ordinary skill in the art are familiar with conventional techniques and technology associated with watermarks, watermark embedding, and watermark detecting.
Robustness
In most watermarking applications, the marked goods are likely to be processed in some way before it reaches the receiver of the watermarked content. The processing could be lossy compression, signal enhancement, or digital-to-analog (D/A) and analog-to-digital (A/D) conversion. An embedded watermark may unintentionally or inadvertently be impaired by such processing. Other types of processing may be applied with the explicit goal of hindering watermark reception. This is an attack on the watermark (or the watermarked good) by a so-called adversary.
In watermarking terminology, an attack may be thought of as any processing that may impair detection of the watermark or communication of the information conveyed by the watermark or intends to do so. Also, an attack may create a false alarm on an un-watermarked content to appear as if it is watermarked. The processed watermarked goods may be then called attacked goods.
Of course, key aspect of a watermarking technology is its robustness against attacks. The notion of robustness is intuitively clear to those of ordinary skill in the art: A watermark is robust if it cannot be impaired without also rendering the attacked goods less useful.
Watermark impairment can be measured by several criteria, for example: miss probability, probability of bit error, or channel capacity. For multimedia, the usefulness of the attacked data can be gauged by considering its perceptual quality or distortion. Hence, robustness may be evaluated by simultaneously considering watermark impairment and the distortion of the attacked good.
False Alarms & Misses
When watermarking, one does not want a high probability of a false alarm. That is when a watermark is detected, but none was inserted into the content by the watermarking agent. This is something like finding evidence of a crime that did not happen. Someone may be falsely accused of wrongdoing.
As the probability of false alarms increases, the confidence in the watermarking technique decreases. For example, people often ignore car alarms because they know that more often than not it is a false alarm rather than an actual car theft.
Likewise, one does not want a high probability of a miss. An event of “miss” happens when watermark is not detected (i.e., declared to be not present) although it was supposed to be detected. This is something like being unable to detect the evidence in a crime scene either by oversight or inability to do so. Because of this, a wrongdoing may never be properly investigated. As the probability of misses increases, the confidence in the watermarking technique decreases.
Ideally, the probabilities of a false alarm and a miss are zero. In reality, a compromise is often made between them. Typically, a decrease in the probability of one increases the probability of the other. For example, as the probability of false alarm is decreased, the probability of a miss increases.
Consequently, it is desirable to minimizes both while finding a proper balance between them.